Information recording devices such as magnetic disks are always required to have a larger recording capacity and to attain a reduction in access time such as disk rotational delay. One possible means for satisfying the latter requirement is to heighten the rotational speed of a medium.
However, since the media are weighed down by themselves, they resonate considerably at an increased rotational speed. Eventually, the surface of such a medium comes into contact with the head to cause an error or crushing. It is therefore impossible to narrow the gap between the magnetic disk head and the recording medium to or below a certain level, and this constitutes a serious obstacle to an increase in recording capacity.
For diminishing the resonance of the medium being rotated, it is necessary to heighten both a modulus of elasticity (Young's modulus) of the substrate of the medium and a rigidity thereof which is the value obtained by dividing the Young's modulus by the density.
Aluminum alloy which has been most commonly used as the substrates of magnetic disks has a modulus of elasticity in the above meaning of 71 GPa and a rigidity as defined above of 26 GPa.multidot.cm.sup.3 /g. This conventional substrate material having such properties hardly copes with the trend toward higher rotational speeds of 10,000 rpm and above. In addition, it has become necessary to increase the thickness of substrates made of the above material from the standpoints of modulus of elasticity (Young's modulus) and rigidity (Young's modulus/density), despite of the current trend toward thickness reduction in disk substrates for device miniaturization.
In contrast, substrates made of a tempered glass are superior to the aluminum substrate in both modulus of elasticity and density. For example, a glass substrate obtained by subjecting a commercially available soda-lime glass to ion exchange in a molten potassium salt is on the market. This substrate has a modulus of elasticity of 72 GPa and a rigidity of 29 GPa.multidot.cm.sup.3 /g.
Also known besides the above one is a glass substrate obtained by tempering commercially available Corning 0317. Although this substrate has a modulus of elasticity of 72 GPa and a rigidity of 29 GPa.multidot.cm.sup.3 /g, these properties are still insufficient.
A high-rigidity substrate for information recording media which is made of a material other than tempered glasses is on the market. This substrate comprises a crystallized glass having a modulus of elasticity of 90 GPa and a rigidity of 38 GPa.multidot.cm.sup.3 /g. However, this substrate, after polishing, inevitably has residual crystal grains projecting from the surface because of the nature of the production process in which crystals are precipitated inside. Namely, this crystallized glass substrate has a drawback that it is inferior in surface smoothness to the substrates made of a tempered glass.
Furthermore, JP-A-10-81542 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a material for use as a substrate for information recording media. This material comprises a glass which is based on SiO.sub.2 -Al.sub.2 O.sub.3 -RO (wherein R is a bivalent metal) and contains at least 20 mol % Al.sub.2 O.sub.3 or at least 20 mol % MgO. However, as apparent from the Examples shown in the above patent document, this prior art material has problems that it has a high liquidus temperature and hence poor moldability, and that it is unsuitable for high-speed rotation because of its high density.
Consequently, in view of the expected future trend toward even higher rotational speeds in information recording devices and smaller thicknesses in disk substrates, there is a desire for a glass composition which has further improved properties, i.e., which has a high modulus of elasticity represented by Young's modulus and a high rigidity (modulus of elasticity/density), can be easily molded, and gives a substrate having high surface smoothness through polishing.